I had a similar situation where I needed email copies to goto different people depending on the customer. I put this email address in the "suburb" field in the main customer address. I use FEC orders so the code below is on line 1067 of includes/classes/order.php.
Code:
zen_mail('', SEND_EXTRA_ORDER_EMAILS_TO . ',' . $this->customer['suburb'], SEND_EXTRA_NEW_ORDERS_EMAILS_TO_SUBJECT . ' ' . EMAIL_TEXT_SUBJECT . EMAIL_ORDER_NUMBER_SUBJECT . $zf_insert_id,
$email_order . $extra_info['TEXT'], STORE_NAME, EMAIL_FROM, $html_msg, 'checkout_extra', $this->attachArray);
The part in red was the only change.